Major applications for a solid-state laser include precision machining of a metal material and a silicon wafer, micro-boring of a printed board material, and the like. In precision machining, such as that mentioned above, a single mode solid-state laser that performs Q-switched high-peak pulse oscillation and a wavelength conversion laser using such a solid-state laser as the fundamental wave laser light source are generally used in order to realize high quality machining by minimizing thermal influence on a material to be machined. Common wavelength conversion lasers include a green laser for which a fundamental laser beam having an infrared wavelength is converted so as to assume a one-half of the wavelength and a ultraviolet laser (hereinafter called an “UV laser”) for which the fundamental laser beam is converted so as to assume a one-third or one-fourth of the wavelength.
A high peak pulse laser is suitable for machining metal and a silicon material among the applications for the solid-state laser, and a high energy solid state laser or wavelength-conversion laser having a short pulse width (a short pulse) has been sought. Since an UV laser having a highly-repetitive frequency is superior in terms of an efficiency in machining a resin material and enables high-speed machining, an UV laser that produces a high average output at the highest possible repetitive frequency has been sought in a range of use for boring a printed board made of a resin material. In order to obtain a high frequency UV laser that produces a high average output, a high frequency pulse solid-state laser that produces a high average output is required as a fundamental laser that serves as the source for the UV laser. Improvements in a technique pertaining to achievement of a shorter pulse and higher energy and improvements in a technique pertaining to achievement of a higher frequency and a higher output have been sought in the field of a transverse single mode solid-state laser. If there is a solid state laser technique satisfying both requests, the technique can be said to be most idealistic.
An Nd:YAG laser using a rod-shaped laser element (hereinafter abbreviated as a “YAG rod”) taking as a material a Y3Al5O12 crystal doped with Nd atoms (popularly called a YAG crystal) and an Nd:YVO4 laser using a YVO4 crystal doped with Nd atoms are common as a solid state layer that performs high power oscillation. Since the Nd:YVO4 layer yields a very high laser gain, the laser is highly efficient and can perform stable pulse oscillation even at a highly-repetitive frequency of the order of 100 kHz. However, difficulty is encountered in growing a large crystal, and hence there is a limit on an increase in power. On the other hand, creation of a large YAG rod is possible in the case of the Nd:YAG laser, and high power, high energy pulse laser oscillation is possible. However, the Nd:YAG laser does not yield as high laser gains as does the Nd:YVO4 crystal, and hence pulse oscillation achieved at a highly-repetitive frequency tends to become unstable. In general, the Nd:YAG laser is made commercially practical at an intermediate repetitive frequency range of 50 kHz or less.
Of current commercially-available UV lasers, UV lasers adopting an Nd:YAG laser as the fundamental laser are predominant in terms of a high energy type, and UV lasers adopting an Nd:YVO4 laser as the fundamental laser are predominant in terms of a highly-repetitive frequency type. However, difficulty is encountered in achieving a further increase in the power of the Nd:YVO4 laser because of restrictions in a crystal size, and the like. If there is available a product technology that enables an Nd:YAG laser, which is easy to generate higher power, to perform stable pulse oscillation even at a highly-repetitive frequency of the order of 100 kHz, a higher-power UV laser can be obtained even in the field of the highly-repetitive frequency.
In order to aim at causing the Nd:YAG laser to perform high power oscillation at a highly-repetitive frequency, an increase in excitation density achieved in the YAG rod is effective. To this end, a resonator technique and an excitation technique, which enable performance of stable laser oscillation in compliance to an intensive thermal lens arising in the YAG rod, are required. The same also applies to production of a high energy, high power Nd:YAG laser. Specifically, even in an attempt to increase the energy and power of the Nd:YAG laser and an attempt to achieve higher repetition and output, a technique for the manner of enabling stable laser oscillation through use of an intensive thermal lens is the key.
A technique for high power transverse single mode oscillation in an Nd:YAG laser will now be described.
The YAG rod has a thermal lens that changes in accordance with an excitation input. Hence, when a resonator is designed, a design value that enables stable oscillation must be determined in consideration of a thermal lens in the YAG rod. Since the thermal lens of the YAG rod changes according to excitation density, the thermal lens of the YAG rod also exhibits a characteristic of a gradual increase according as the excitation input is changed from a low input to a high input. However, there is a limit on the intensity of the thermal lens held one resonator for enabling continued stable oscillation, and laser oscillation can be performed within only the range of a specific thermal lens. The range of the thermal lens has an upper limit and a lower limit, and values of the upper and lower limits and the size of the range of thermal lens changes according to the design value of the resonator. Specifically, the resonator designed for requirements of a less-intensive thermal lens cannot perform laser oscillation by means of an intensive thermal lens (=a high excitation input). Further, a resonator designed for an intensive thermal lens cannot perform laser oscillation through use of a less-intensive thermal lens (=a low excitation input). The range of thermal lens where laser oscillation can be performed is hereinafter called an oscillation range for a resonator.
The size of the oscillation range changes according to the design value of the resonator, and is determined primarily by a ratio of a cross-sectional area of an excited area in a YAG rod taken along a direction perpendicular to an optical axis to a cross-sectional area of a TEM00 beam generated within the YAG rod theoretically computed from the design value of the resonator. For instance, in the case of an Nd:YAG laser of lateral pumping type, the entirety of a YAG rod is excited, and hence the size of the oscillation range is determined by a ratio of the cross-sectional area of the TEM00 beam to a circular cross section of the YAG rod. Therefore, in the case of lateral multimode oscillation in which oscillation is induced at the diameter of a TEM00 beam that is sufficiently smaller than the diameter of the YAG rod, a wide oscillation range is assured. However, in the case of transverse single mode oscillation in which oscillation is induced at the diameter of a TEM00 beam close to the diameter of the YAG rod, only a narrow oscillation range can be assured. It is also possible to prevent oscillation of a multimode component by inserting an aperture that is sufficiently smaller than the diameter of the YAG rod into the resonator, thereby inducing transverse single mode oscillation at a beam diameter that is sufficiently narrower than the diameter of the YAG rod. In such a case, a wide oscillation region can be assured; however, laser oscillation is forcefully performed at an extremely low efficiency, so that high-power oscillation cannot be carried out. The same also applies to the case of end pumping in which the center of an end face of a YAG rod is intensively excited. In a case where there is designed a resonator which induces transverse single mode oscillation at the diameter of a TEM00 beam equivalent to an excitation range, the width of the oscillation region becomes narrow accordingly.
In the event, when a highly-efficient, high-power transverse single mode oscillator is designed, a narrow oscillation region is inevitably, forcefully adopted without regard to the excitation method, whereupon there is obtained a laser that causes oscillation in only a specific, narrow range of excitation input and that exhibits an input-output characteristic having a sharp geometry. In the case of lateral multimode oscillation exhibiting a low light convergence characteristic, a wide oscillation region can be assured, and there is obtained a high power laser that exhibits a broad input-output characteristic of inducing oscillation from a low excitation input level to a high excitation input level. However, in the case of high power transverse single mode oscillation, a laser can also be said to exhibit a narrow, sharp input-output characteristic in exchange for a high light convergence characteristic.
Incidentally, when the width of the oscillation region is extremely narrow, a characteristic susceptible to fluctuations and variations in thermal lens of the YAG rod is exhibited, which hinders performance of stable laser oscillation and makes it impossible to expect high power oscillation. Although there is a limit on the width of an oscillation region required to induce high power oscillation, the width of the oscillation region usually becomes smaller than the limit value when an attempt is made to induce transverse single mode oscillation at high efficiency. Even when a resonator that induces oscillation at a high excitation input is designed, an output saturation phenomenon arises, so that intended high power oscillation cannot be induced. A symmetrical resonator configuration, in which right and left resonator mirrors are imparted with the same curvature and arranged in the layout of a symmetrical optical system, is effective for avoiding the limit. In the case of an asymmetrical resonator configuration, two narrow oscillation regions are separately present in a low-excitation input side and a high-excitation input side, respectively. Therefore, the respective oscillation regions are narrow, and an output saturation phenomenon arises. However, in the case of the symmetrical resonator configuration, the two oscillation regions are not separated and create as one oscillation region having a twice size. Therefore, sufficient stability is achieved in defiance of fluctuations and variations in thermal lens, and high power transverse single mode oscillation compliant with an increase in excitation input can be induced.
Conditions of a thermal lens under which a resonator performs laser oscillation is principally designed on the basis of design values, such as a curvature of a resonator mirror and a mirror layout. When occurrence of oscillation under conditions of an intensive thermal lens (a high excitation input) is desired, it is common to realize a symmetrical resonator configuration by use of resonator mirrors having high curvatures (i.e., short curvature radii). However, when oscillation is induced under conditions of an extremely-intensive thermal lens, the size of a beam achieved in the resonator mirror may become extremely small, and a light-resistance limit on a coating of the mirror may be exceeded, to thus inflict damage on the mirror. In order to avoid such a limit, there is also available a configuration in which a thermal lens compensation optical system (made up of a concave lens, and the like) which cancels a thermal lens of a YAG rod is built into a resonator, to thus enable laser oscillation even under conditions of a further intensive thermal lens.
In the meantime, the YAG rod encounters a problem of double thermal lenses; namely, occurrence of two different types of thermal lenses according to the direction of polarization. The problem is attributable to occurrence of birefringence in an excited YAG rod as a result of generation of stress associated with the distribution of temperature, and corresponds to a phenomenon of inducing different thermal lenses for two types of light beams having planes of polarization in a radial direction and a circumferential direction of the cross section of the YAG rod. As a consequence, since two types of polarization modes induce oscillation in states of different thermal lenses within the resonator, competition arise between the modes. In particular, in the case of transverse single mode oscillation, there arises a problem of deterioration of oscillation efficiency and occurrence of an extremely-unstable oscillating state.
A birefringence compensation technique is effective for solving the problem, wherein a 90 degree polarization rotator, such as a quartz rotator, is interposed between two equivalently-excited YAG rods, thereby averaging the total of thermal lenses sensed during the course of two polarized modes of beams making a round trip within the respective resonators. Highly-efficient transverse single mode oscillation becomes possible as a result of oscillation being stably induced in the state of the thermal lenses having the same polarized mode.
FIG. 1 of Patent Document 1 illustrates an example related-art resonator using the birefringence compensation technique. Moreover, in this example, there is provided a resonator configuration in which the foregoing thermal lens compensation means built from the concave lens is disposed outside both rods, to thus enable high power oscillation by means of further intensive thermal lenses.
A method known as means for performing more perfect birefringence compensation is a combination of the birefringence compensation technique with a technique for establishing an image transfer link between the YAG rods by means of a telescope. In addition to describing the birefringence compensation technique using the 90 degree polarization rotator, FIG. 5 of Non-patent Document 1 describes a configuration in which a telescope consisting of two lenses (having a focal length “f”) is positioned between the two YAG rods on the assumption that an optical distance between the center of the YAG rod and the lens is defined as “f” and that an optical distance between the two lenses is defined as 2 f. There is provided an example in which the influence of the double thermal lenses is completely eliminated by the configuration, to thus implement transverse single mode oscillation of linearly-polarized light having a maximum output of 114 W.
Like a configuration shown in FIG. 14 of Non-patent Document 2, there is also a method for producing a high power transverse single mode output by combination of a MOPA technique for causing a transverse single mode laser beam oscillated by the birefringence compensation technique to pass through excited YAG rods and outputting an amplified laser beam.
The wavelength conversion laser technique will now be described.
In general, in a high power wavelength conversion laser typified by a UV laser, light output from a solid-state laser oscillated by the linearly-polarized light is caused to enter nonlinear crystal as a fundamental laser beam, whereupon the laser beam is output after being converted into a harmonic laser beam having a 1/N wavelength. Common non-linear crystals include an LBO crystal (LiB3O5), a KTP crystal (KTiOPO4), a CLBO crystal (CsLiB6O10), and the like. A laser beam converted so as to assume a half wavelength is called a duplicate harmonic; a laser beam converted so as to assume a one-third wavelength is called a triple harmonic; and a laser beam converted so as to assume a quarter wavelength is called a quadruple harmonic. A linearly-polarized laser oscillator of transverse single mode pulse-oscillated by means of a Q switch is used as a fundamental laser. As mentioned above, in addition to a request for an increase in energy and output, there is a request for an increase in frequency and output particularly for an UV laser typified by a third harmonic. To this end, a fundamental laser of transverse single mode that stably oscillates laser even at a highly-repetitive frequency is indispensable. Under present circumstances, there are many UV laser products adopting, as a fundamental wave, an Nd:YVO4 laser advantageous for oscillation at a highly-repetitive frequency. For instance, a third harmonic laser having an average output of about 20 W at a repetition frequency of; for instance, 100 kHz, is also commercially available. In the meantime, a UV laser of high energy type often adopts an Nd:YAG laser advantageous for an increase in output as a fundamental laser.
FIG. 14 of Non-patent Document 2 shows a report about an example in which an output of a fundamental laser beam oscillated by a birefringence compensation technique, such as that mentioned above, is increased to 205 W by the MOPA technique that causes the fundamental laser beam to pass through an excited YAG rod, to thus produce an amplified output, and that causes the thus-amplified output to enter nonlinear crystal, whereby a third harmonic having a maximum output of 64 W is acquired at a frequency of 40 to 45 kHz.
Patent Document 1: JP-A-2003-8121 (FIG. 1)
Non-patent Document 1: M. Frede et al., “High power fundamental mode Nd:YAG laser with efficient birefringence compensation,” Opt. Express 12, 3581 to 3589 (2004) (FIG. 5)
Non-patent Document 2: Charles X. Wang et al., “High Power Q-switched TEM00 Mode Diode-Pumped Solid State Lasers with >30 W Output Power at 355 nm,” Proc. Of SPIE Vol. 6100, 610019, (2006) (FIG. 14)